The present invention relates to turbochargers in general, and more particularly relates to high pressure ratio turbochargers employing a two-stage compressor having first- and second-stage impellers arranged in series.
Developments in the turbocharger field continue to require increased pressure ratios for providing improved fuel economy, higher power ratings, and improved emissions performance for engines on which turbochargers are employed, particularly for commercial diesel application. With conventional turbocharger designs, the typical method for achieving such increased pressure ratios has been to increase the rotational speed of the compressor and turbine components. Current pressure-ratio capability for turbochargers of conventional design is typically in the 3.5 range, although some specialized designs can operate at about 4.0. Currently, the only known method for increasing the pressure-ratio capability of a compressor, for a given maximum rotational tip speed, is to reduce the backward curvature of the blades. Backward curvature is used to improve the flow-range capability of a compressor as well as to improve the efficiency, and thus reducing the backward curvature results in less efficiency and a narrower flow range. Requirements for commercial diesel engines for trucking and industrial applications are rapidly approaching pressure ratios of 5 to 6 and possibly higher with flow ranges of over 2.5:1 choke flow to surge flow ratio. Material property limits are exceeded in the rotating components of conventional turbocharger designs at these pressure ratios due to the stresses imposed by the required high rotational speeds. For a turbocharger using a traditional single-stage compressor design, the optimum turbine design for efficiency cannot be used because of the high inertia of a low specific-speed design. High inertia reduces the response of the turbocharger to meet the transient requirements of the engine.
Multiple-stage compression through the use of two or more turbochargers operating with their compressors in series has been an approach to meeting elevated pressure-ratio requirements. However, the cost and complexity of such systems as well as the packaging size requirements are unattractive for most applications.
Turbochargers have been produced having a two-stage compressor in which two impellers are mounted on the same shaft. The compressor housing is configured to route air first through one impeller and then through the other before supplying the air to the engine air intake system. With such two-stage serial compressor designs, pressure ratios of 5 or greater can be achieved at reasonable rotational speeds.